Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). Also, CdTe converts radiation energy in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in cloudy conditions as compared to other conventional materials. The junction of the n-type layer and the p-type layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type layer (i.e., a positive, electron accepting layer) and the CdS layer acts as a n-type layer (i.e., a negative, electron donating layer).
During processing of cadmium telluride based PV devices, the window layers (e.g., the cadmium sulfide layer) are typically deposited prior to the cadmium telluride layer. However, the cadmium sulfide layer is susceptible to exposure to atmospheric contamination prior to deposition of subsequent layers (e.g., the cadmium telluride layer). However, exposing the cadmium sulfide layer to air can adversely affect the interface between the cadmium sulfide layer and subsequent layers (e.g., a cadmium telluride layer) by introducing contaminants to the cadmium sulfide layer. Additionally, during the deposition of a subsequent cadmium telluride layer on the cadmium sulfide layer via a hot deposition process (e.g., close space sublimations), cadmium sulfide can sublimate from the surface of the substrate due to cadmium sulfides lower sublimation temperature. Thus, the cadmium sulfide layer must be deposited to a greater thickness than desired in order to compensate for the loss of the layer during subsequent processing. Additionally, the sublimation of the cadmium sulfide layer can add compositional additions of cadmium and sulfur to the subsequent layer (e.g., a cadmium telluride layer). The heating process during deposition of subsequent layers can also alter the as-deposited morphology of the cadmium sulfide (e.g., amorphous or fine grain size).
Thus, a need exists for a method of protecting the cadmium sulfide layer during the manufacture of a cadmium telluride based PV device without adversely affecting the resulting PV device.